Just in case you haven't visited a record store in the last few months, the news is simple and almost surreal: Ol' Blue Eyes is back-again. Many reasons have been given for the resurgence of interest in Frank Sinatra, among them: (1) the commercially astute pairing of the 78-year-old institution with younger pop stars on Duets; (2) the aggressive marketing of said album, for which his label, Capitol, flooded the market with nearly 1.3 million copies before its release last November; (3) the newfound appeal of old-world crooners like Sinatra and Tony Bennett with the twentysomething crowd, who find plenty of novelty value in old guys in tuxes singing songs from their grandmothers' favorite movies; and (4) pure, unadulterated camp of the sort that recently reactivated, for a brief moment, the flagging career of Tom Jones. ''It has to do with the whole mystique of Frank Sinatra,'' says Bill Ryan, owner of Pier Platters, a record store that happens to be in Sinatra's hometown (Hoboken, N.J.) and that caters to the alternative- and college-radio listeners who have been buying Duets. ''They think he's cool, and he did duets with people they like. That combination adds up to 'Let's buy it.''' And buy it they have: To date, 1.8 million people have snapped up copies of Duets, which has held its own on the pop charts alongside albums by toddlers like Pearl Jam, Snoop Doggy Dogg, and Mariah Carey. There is some truth to all of these theories, but they don't get to the heart of the matter. The appeal of Sinatra in the alternative-rockin' '90s is much subtler and simpler: He is the original gangsta rapper. Consider the evidence: *No one before-and few since-inspired fear and intimidation in mortals the way Sinatra did. Swaggering, seemingly afraid of nothing, walking it like he talked it, and undeniably talented, he set the stage for the likes of Ice Cube and Dr. Dre. *He invented the idea of the posse. Back in the '50s, it had a different name- the Rat Pack, consisting of martini-sodden buddies Dean Martin, Sammy Davis Jr., Peter Lawford, and Joey Bishop. With them, Sinatra could indulge in assorted libations and talk dirty about women. Transpose that to pot and this decade, and you have Dr. Dre's posse. *He uses words like broad and chick and, for female reporters, ''buck-and-a- half hookers.'' It's just a short drive-by from those phrases to bitch or ho, favored terms of endearment among gangsta rappers. *He's been known to assault people he doesn't like, namely reporters or photographers who cross him. Or he has someone else assault them for him. Dr. Dre, the triple-platinum gangsta-rap warlord accused of battery, would be proud. *Sinatra lovingly called his friends ''rat bastards.'' Gangsta rappers lovingly call their friends ''niggaz.'' *He was raised on the streets. Granted, the 'hood was Hoboken, and supposedly it was not as scrappy as legend would have it. Still, that up-from-poverty tale is as much a part of his legend as it is of Snoop Doggy Dogg's. *A born-again Republican, Sinatra has been invited to the White House, paving the way for rapper/Republican Senatorial Inner Circle party guest Eazy-E. *A devout Catholic, he professes to be deeply religious and spiritual. Snoop thanked God on his new album too. If those comparisons sound like a stretch, it's only because age has tamed Sinatra. No longer the arrogant, sharp-brimmed presence of yore, Sinatra in 1994 is a quaint pop Mount Rushmore. He plops himself onto concert stages, his confidence and charisma intact, but the edge is gone: Staring at TelePrompTers to remember lyrics and introductions, he sings in a voice gone gruff and reedy. Industry wags often wonder whether he's somewhat out of touch with reality, and the stories aren't helped by such incidents as Sinatra's making a video for Duets and reportedly referring to his partner, the lead singer of U2, as ''Botto.'' No matter. All of it-the rumors, the gossip, the sniping of music critics- washes over Sinatra. Some of that longevity has to do with his music (see discographies on pages 42 and 44). He has clearly survived creative lulls- there are some mid-'60s albums you should avoid altogether-but it's remarkably easy to select just about any of his eras, toss on a CD, sit back, and revel in exquisite arrangements, music that swings or broods, and a voice with enough snap, crackle, and sob to make it seem as if he lived every word he sang. Even on some of Sinatra's campier '60s records, that voice is rugged, lean, and tough, with character and elegance to burn.